In thermometers, particularly fever thermometers for adult people, for children and for animals, accuracy is required. Also, the response should be as rapid as possible. Further, the expense of purchase, maintenance and cleaning a thermometer should be as low as possible. Particularly in a hospital, physician's office, other medical facility or veterinary facility, the foregoing factors are quite important.
Cleaning and more particularly sterilizing a thermometer after each use is time consuming and can be expensive. For example, with a standard mercury bulb thermometer, heat sterilization might destroy the thermometer, while other forms of sterilization, like gas sterilization, are relatively expensive. Conventional sterilization techniques are not always as effective as they might be. This can result in reinfection of a patient by a supposedly sterilized thermometer which is still carrying the organisms that had infected that patient and for which that patient was being treated and, on occasion, can cause infection of a patient with organisms from a prior patient.
Repeat use or non-disposable thermometers must be sturdy and strong enough to withstand the mechanical stresses of normal use. The temperature sensing element, such as a mercury bulb or other mechanical element, is accordingly larger and correspondingly slower to respond to a sensed temperature change. For example, there is now available a non-disposable thermometer comprised of a temperature responsive thermistor which is electrically connected in a permanent manner to a temperature indicating meter. This unit and particularly the sensing element thereof must be large enough to not be damaged or destroyed in use.
To overcome the foregoing problems, various relatively inexpensive fever thermometers have been developed, which are disposable in their entirety, or at least that part of the thermometer to which the patient is exposed is disposable. Two examples of single use, completely disposable thermometers, which were developed by the inventor hereof, are shown in U.S. Pat. Nos. 3,379,063 and 3,507,154. It is one goal of the inventor hereof to simplify the fever thermometer even further, make it even less expensive to manufacture, and therefore make it even more practical in use.
An example of a partially disposable thermometer that is presently used comprises a relatively thick temperature-sensing probe over which a replaceable sheath is positioned. The sheathed probe is placed in the patient's mouth for a short time. The patient's body heat reaches the sensing element in the probe through the sheath. Because of the size and volume of the probe and the short duration of the temperature sensing procedure, the temperature of the temperature sensing element in the sheathed probe never rises to the actual temperature of the patient. Therefore, the probe is electrically connected to a relatively expensive predictor circuit which translates the rate of change of temperature of the probe into a digital readout of the actual temperature of the patient, i.e., the circuit predicts what temperature the probe would sense were the probe to finally be heated to the patient's body temperature.
Another problem with a thermometer, and particularly a fever thermometer, is the size and volume of the temperature sensing element and/or the housing or sheath over the temperature sensing element. Usually, this housing or sheath must be heated by the body of the person whose temperature is being sensed, at least in the vicinity of the temperature sensing element, before the temperature sensing element can accurately sense the body temperature. For example, in a standard mercury bulb fever thermometer, the thermometer must be held in place in the patient's body a sufficient time to heat the glass around the mercury bulb and the glass near the mercury bulb, as well as the mercury in the bulb. In the present inventor's prior U.S. Pat. No. 3,507,154, the temperature-sensing element comprises a heat expansible coil in a housing. The thermometer must remain in place in the patient's body at least long enough to heat the entire coil and the housing around the coil. In the above described thermometers which used a heat sensing probe connected to a predictor circuit, because the probe is subject to repeated use, it must be sufficiently mechanically strong. This requires the probe be of relatively large volume and the probe consequently takes longer to heat up, thereby necessitating use of the relatively expensive predictor circuit.
Further, the thicker or heavier the casing and the more temperature conductive it is, the longer the time that elapses before the body whose temperature is being measured heats the temperature sensing element in order that an accurate temperature reading might be taken. Accordingly, it is desirable to minimize the thickness of the housing over the temperature sensing element. (With respect to sensors of other types of energy, or the like, it is similarly desirable to make the casing thinner or, as appropriate, more pervious to the energy or condition being sensed.) In this manner, the thermometer will provide a more rapid reading of the temperature.
Relatively large thermometer housings create a comfort and convenience problem. Body temperature is preferably taken at a location in the body near to arterial blood flow. In a human being, arterial blood flow is near a surface of the body at the end of the rectum, beneath the tongue and under the arm. The larger the size, thickness and volume of the housing of the temperature sensor, the more uncomfortable will be its emplacement and holding in position in the body near arterial blood flow. Conversely, the smaller these dimensions of the housing of the temperature sensor, the more comfortable it will be to keep the thermometer in place. In taking temperatures from small children and animals, for example, accurate readings are difficult to obtain because the patient is made uncomfortable by the emplacement of the thermometer, usually in his rectum, and the squirming of the patient makes it difficult to hold the thermometer in place for the desired minimum time period. If the fever thermometer can be made sufficiently small that the patient will hardly feel its insertion or its presence once inserted and if the time for a temperature taking can be shortened, the patient will not be made uncomfortable and a more accurate temperature reading will be obtainable.
Thermometers, once manufactured, must have an adequate shelf life and must not be damaged by ambient conditions to which they might be exposed following manufacture, including the shocks of transportation, of being placed in and removed from storage, including the relatively rough handling during setup and use, and including the temperature and humidity, and the like conditions, of a storage place. Conventionally available thermometers are sufficiently rugged and protected to withstand normally anticipated ambient conditions. But, when the size and volume of a thermometer is reduced and the sheathing over the temperature sensing element is made thinner and lighter, then a thermometer becomes more delicate and more easily damage by normally encountered and ambient conditions.